Understanding of ribosome function has been a goal of molecular biology for more than 50 years. Of particular interest has been the mechanism by which the ribosome achieves translational accuracy, and how accuracy is perturbed by highly effective antibiotics. Rules for how ribosome distinguishes correct and incorrect decoding events have been developed. The work proposed here seeks a broader understanding at this level using a novel approach. A unique RNA molecule, tmRNA , and its intreraction with the ribosome will be examined. TmRNA has features of both a tRNA and an mRNA and functions to terminate translation of stalled ribosomes. In order to carry out its function in the ribosomal A site, a tmRNA is proposed to mimic the codon-anticodon interaction of the normal tRNA mRNA interaction. The proposed research will utilize a sensitive genetic screen to identify tmRNA mutants that have reduced or increased function in vivo, characterize those mutants at the molecular level to identify determinants recognized by the ribosome, and examine promising candidates, genetically and with key antibiotics to determine if they are defective in responding to the rules for accurate decoding by the ribosome. A deeper understanding of these processes will not only reveal new functional aspects of both the tmRNA and the ribosome but likely yield new insights into how antibiotics function and how new antibiotics might be discovered.